TALLINN  Estonia's warmest winter for two centuries has woken some of its 600 bears several months early from hibernation, wildlife experts say.

The bears' early reappearance has raised concerns for the survival of this year's cubs.

"It has been very warm and wet and many flooded rivers have forced bears out of their dens and out of hibernation," said Kalev Manniste, a senior official at the Baltic country's State Forest Service.

"Just a few days ago a hunter was telling me that he saw a she-bear with a very small cub walking across the field," he told Reuters.

"The cub the hunter saw looked too small to survive the winter."

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She-bears normally give birth to tiny walnut-sized cubs during their winter hibernation and suckle them for months as they grow, before the spring thaw awakens the mother and she leaves her den.

Local media and hunters writing on Internet sites say that across the country bears are moving about the forests at a time when they normally sleep and would not be seen for another two to three months.

Temperatures have stayed above freezing, compared with the average temperature of minus 5 Celsius (23.00F) for January.

Neighbouring Russia's normally ferocious winter has also been mild. Interfax news agency reported this week that a bear in a zoo awoke from hibernation two months early, while another did not go to sleep at all.